334 Minnesota Plant Diseases. 



very common upon plants raised on previously infected land 

 and in such cases infection is almost certain. The disease 

 spreads rapidly and may cause great damage in a single season. 



Infected plants should be destroyed to prevent future infec- 

 tions. When a field is badly infected, as when infected plants 

 are left on the field, the rotation of crops becomes necessary. 

 Spraying with bordeaux has given satisfactory results in com- 

 bating this disease. In some cases this has proved injurious to 

 the foliage when used in standard strength. There is also diffi- 

 culty in making the bordeaux adhere to the very glossy surface 

 of the onion leaf. Powdered quicklime (two parts) and sul- 

 phur (one part) has been recommended for checking the dis- 

 ease in the early stages. Spraying with potassium sulphide is 

 also effective. Of chief importance are the avoidance of too 

 damp conditions and too much shade, the removal of infected 

 plant parts and the rotation of crops, if necessary. 



Downy mildew of cucumber, melon and other gourds [Plas- 

 mopara cubensis (B. & C.) Humpf.]. This has proven in certain 

 parts of the United States to be an exceedingly virulent disease 

 and has threatened to completely destroy the pickle crops in 

 those districts. It is not as yet known to be common in Min- 

 nesota, but it will probably appear here in time and the farmer 

 who is employed in the growing of cucumbers of other gourd 

 fruits will do well to know the disease and to keep a sharp look- 

 out for it. It is known to attack cucumbers, muskmelons, 

 pumpkins, warty and winter squash, watermelons and various 

 other gourd fruits. Unlike the powdery mildew of the cu- 

 cumber, it is more common in fields and gardens than in green- 

 houses, but it is not unknown under the latter conditions. It 

 is in many places the chief enemy of cucumber culture. An 

 important feature lies in the fact that it can spread from one 

 host to another. 



The disease is best recognized by the action upon the 

 foliage, though it is by no means confined to the leaves, 

 but may occur on the stem as well. Infected leaves turn 

 yellow in spots and these yellow spots are bounded by the veins 

 of the leaves and are therefore usually four-sided and angular. 

 They are up to a quarter of an inch across, but by joining with 

 neighboring spots may become much larger. The spots in- 



